Method of producing direct positive images with photographic silver halide material containing compound releasing iodide ions

ABSTRACT

IN A KNOWN METHOD OF PHOTOGRAPHICALLY PRODUCING DIRECT POSITIVE IMAGES BY EXPOSING A LIGHT-SENSITIVE MATERIAL CARRYING A SILVER HALIDE EMULSION LAYER OF THE TYPE FORMING ALATENT IMAGE PREDOMINANTLY IN THE INNER PART OF THE SILVER HALIDE GRANIS, I.E. A SO-CALLED INTERNAL IMAGE EMULSION, TREATING THE EXPOSED MATERIAL WITH A SURFACE DEVELOPER AND OVERALL EXPOSING THE MATERIAL DURING THIS TREATMENT TO ACTINIC LIGHT OF LOW INTENSITY, AN IMAGE HAVING IMPROVED DENSITY AND CONTRAST IS OBTAINED BY INCORPORATING WITHIN THE EMULSION LAYER A COMPOUND RELEASING IODIDE IONS IN AN AQUEOUS MEDIUM. THE CONCEPT CAN BE APPLIED TO THE FORMATION OF BLACK-AND-WHITE IMAGES AS WELL AS COLORED IMAGES.

2, 1973 G. L. VANREUSEL 3,708,298

METHOD OF PRODUCING DIRECT POSITIVE IMAGES WITH PHOTOGRAPHIC SILVER HALIDE MATERIAL CONTAINING COMPOUND RELEASING IODIDE IONS Filed Feb. 1, 1971 CUR V5111 DENSITY United States Patent 3,708,298 METHOD OF PRODUCING DIRECT POSITIVE IMAGES WITH PHOTOGRAPHIC SILVER HALIDE MATERIAL CONTAINING COM- POUND RELEASING IODIDE IONS Gerard Laurens Vanreusel, Hove, Paul Dsir Van Pee, Edegem, and Jules Maria De Laet, Mortsel, Belgium, assignors to Agfa-Gevaert, Mortsel, Belgium Continuation-impart of applications Ser. No. 677,519, Oct. 24, 1967, now abandoned, and Ser. No. 110,635, Jan. 28, 1971, said application Ser. No. 110,635, being a continuation of application Ser. No. 546,640, May 2, 1966, now abandoned, and said application Ser. No. 677,519 being a continuation-in-part of said application Ser. No. 546,640. This application Feb. 1, 1971, Ser. No. 111,705 Claims priority, application Great Britain, Apr. 30, 1965, 18,335/65 lint. Cl. G03c 5/24 US. or. 96-64 33 Claims ABSTRACT OF THE DISCLOSURE In a known method of photographically producing direct positive images by exposing a light-sensitive material carrying a silver halide emulsion layer of the type forming a latent image predominantly in the inner part of the silver halide grains, i.e. a so-called internal image emulsion, treating the exposed material with a surface developer and overall exposing the material during this treatment to actinic light of low intensity, an image having improved density and contrast is obtained by incorporating within the emulsion layer a compound releasing iodide ions in an aqueous medium. The concept can be applied to the formation of black-and-white images as well as colored images.

This application is a continuation-in-part of application Ser. No. 110,635 filed Jan. 28,- 1971, as well as of application Ser. No. 677,519 filed Oct. 24, 1967, now abandoned. Application Ser. No. 110,635 is, in turn, a streamlined continuation of application Ser. No. 546,640 filed May 2, 1966, and now abandoned, while Ser. No. 677,519 is a continuation-in-part of the same application Ser. No. 546,640.

The present invention relates to a novel method of producing photographic direct positive images and more particularly to a simple method according to which direct positive images of good quality are produced rapidly.

In German patent specification 749,864 a method is described of producing a photographic direct positive image that comprises the steps of exposing a silver halide emulsion layer to an object or image, predeveloping the thus exposed layer for a considerably long time, subjecting it to an overall exposure with actinic light of low intensity and finally developing the said emulsion layer to cause a direct positive image to be formed. The said predeveloping occurs preferably in a fine grain developer or fiat-working developer and the said developing in an energetic developer. The silver halide emulsion layer for use in the process of said German patent specification must be such that its negative image sensitometric curve shows a maximum density that does not exceed the value 0.5.

In British patent specification 581,773 a method is described of producing a photographic direct positive image that comprises the steps of exposing to an object or image a light-sensitive silver halide emulsion layer, treating said emulsion layer to render it capable of forming a surface latent image and then subjecting it to a uniform exposure of sufiicient intensity and time to cause a direct positive 3,708,298 Patented Jan. 2, 1973 'ice image to develop by treating it in a developer that will develop the surface latent image but will not or only slightly develop the first formed internal latent image.

The silver halide emulsion layer used must be of the type in which the latent image is formed mostly or entirely inside the grains. According to the teaching of said British patent specification the light-sensitive silver halide emulsion layer that has been exposed to an object or image may also, without development of the latent image so formed to a visible image be dipped in a photographic developing solution, which is capable of developing a surface latent image, but is capable of developing the latent image formed by said exposure only slightly, if at all, whereupon the emulsion layer is subjected to a uniform exposure of sufiicient intensity and time to cause a direct positive image to be developed when the treatment is continued in the same or another surface developer.

The above methods of producing photographic direct positive images do not lead to the production of good quality images, particularly because the minimum density of the direct positive image is not low enough and because, at least according to the method of the said British specification, no high maximum density in the shoulder of the direct positive sensitometric curve can be attained. Moreover, the method of said German specification is rather cumbrous. The fact that the minimum density is not low enough is due to an interference of the negative image, which forms in the silver halide emulsion layer in addition to the direct positive image, with the latter image.

It has now been found to produce a photographic direct positive image with a high maximum density and a suflicient low minimum density by a method comprising the steps of:

image-wise exposing a light-sensitive material, which comprises a layer of a silver halide emulsion of the type that forms a latent image predominantly in the inner part of the silver halide grains;

treating the light-sensitive material thus exposed with an aqueous energetic surface developing liquid; and

overall exposing the light-sensitive material during the treatment stage to actinic light having an intensity, which is lower than a normal daylight intensity,

wherein a compound releasing iodide ions in an aqueous medium, this compound being other than silver iodide, has been incorporated within said emulsion layer subsequent to precipitation of the silver halide of said emulsion.

In order to stabilize the photographic direct positive image obtained after said treating and overall exposure steps the material is generally fixed, e.g. in a usual fixing solution such as a thiosulphate solution and washed, whereupon the material may be dried or glazed.

Thus, the present invention provides a method of producing direct positive images of good quality in a rapid and simple way. This method is particularly suitable for producing continuous tone direct positive images but it is also appropriate for reproducing line originals.

The light-sensitive material of use according to the present invention generally comprises a support, for instance a paper support or a hydrophobic film support that is either transparent or not, to which the light-sensitive silver halide emulsion layer is applied, if need be by means of an appropriate subbing layer.

The silver halide emulsion used in carrying out the method of this invention is of the type in which the latent image is formed predominantly in the inner part of the silver halide grains. This means an emulsion only few or none of the exposed grains of which are developable to silver by a developing solution which cannot act as a developer for latent image inside the grains, i.e. a socalled surface" developer such as the following developing solution:

p-Hydroxyphenylglycine g 10 Sodium carbonate (cryst.) g 100 Water to cc 1000 and the exposed grains of which are well developable to silver by a developing solution which acts as a developer for latent image inside the grains, i.e. a so-called internal Any reference in the description and in the claims to a silver halide emulsion of the type that forms a latent image predominantly in the inner part of the silver halide grains is limited to a silver halide emulsion a test layer of which, upon exposure to a light intensity scale for a fixed time between 1/ 100 and 1 second and development for 3 minutes at 20 C. in the above internal developer, exhibits a maximum density at least 3 times and preferably at least times the maximum density obtained when an identical test layer of the said silver halide emulsion is equally exposed and then developed for 4 minutes at 20 C. in the above surface developer. Said preference for a maximum density of at least 5 times as large particularly applies to the case the method of the invention is applied to the production of black and white direct positive images. When the method of the invention is applied to the production of color images wherein the formed silver image is eliminated as will be explained detailedly further in the description, 3 times said maximum density will suflice in order to obtain good results.

The silver halide emulsions used according to the invention are generally not or only slightly chemically ripened silver halide emulsions, since the extent of the surface latent image forming ability increases with the degree of chemical ripening.

Silver chlorobromide emulsions comprising at least 20 mole percent of silver bromide and pure silver bromide emulsions have proved to be specially suitable for the purpose of the invention. These emulsions may contain up to 5 mole percent of silver iodide relative to the total amount of silver halide. Washed as well as unwashed silver halide emulsions may be used.

The silver halide emulsions for use in the present process are generally gelatino silver halide emulsions. However, the gelatin may at least partly be replaced e.g. by another protein, a hydrophilic not proteinaceous colloid, e.g. a polyvinylpyrrolidone or a synthetic polymer applied from an aqueous dispersion, i.e. a latex, e.g. a polyethyl acrylate latex. The presence of such other binders often has also a favourable photographic effect on the formation of the direct positive image. For instance, the addition of the polyvinylpyrrolidone and of said polyethyl acrylate latex in most cases increases the maximum density of the direct positive image.

The light-sensitive silver halide emulsions used according to the invention may be prepared according to any technique known in the art. A method according to which emulsions are prepared, which have proved to be particularly suitable for the purpose of the invention, is the so-called conversion method. According to this method a more soluble silver halide is converted in a less soluble silver halide, e.g., a silver chloride emulsion is converted in the presence of water-soluble bromide and occasionally iodide, the amounts of which are adapted in view of the final composition aimed at, into a silver chlorobromide or a silver bromide emulsion which may occasionally comprise small amounts of silver iodide. This conversion is carried out preferably very slowly by several consecutive steps. Another technique according to which suitable emulsions for use according to the invention may be obtained has been described in the British patent specification 1,011,062. For further specifications of the nature and preparation of light-sensitive silver halide emulsion layers suitable for use according to the present invention reference is made to the specific examples given below.

Into the silver halide emulsion layer a compound releasing iodide ions in an aqueous medium, this compound being other than silver iodide, is incorporated subsequent to precipitation of the silver halide. The presence of this compound causes the favorable effect of the invention, viz the production of a direct positive image showing a high maximum density. According to a preferred embodiment the compound releasing iodide ions is added to the liquid silver halide emulsion after the silver halide grains have been precipitated.

If the silver halide emulsion is prepared according to the conversion method described above, the compound releasing iodide ions may not be added earlier than after the said conversion has been carried out. The compound releasing iodide ions may also be incorporated within the emulsion layer by diffusion from or through another layer, which is in water-permeable relationship with the emulsion layer. This diffusion occurs automatically and immediately when the silver halide emulsion is applied to a water-permeable layer containing the compound releasing iodide ions or when an aqueous coating liquid containing the compound releasing iodide ions is applied to the silver halide emulsion layer or to a water-permeable layer present on top of the emulsion layer. In any case the compound releasing iodide ions must be present in the silver halide emulsion layer before the aqueous energetic surface developer is applied to the imagewise exposed light-sensitive material. The favorable eifect of the method of the invention cannot be obtained if the compound releasing iodide ions is present in the developer itself. Proceeding in this way is outside the scope of this invention.

The compound releasing iodide ions is generally incorporated within the silver halide emulsion layer at a concentration within the range of about 0.01-20 grams per mole of silver halide and preferably in an amount of between 0.1 and 5 grams per mole of silver halide.

Suitable compounds releasing iodide ions are watersoluble iodides, inorganic as Well as organic iodides, organic compounds with labile iodine atom, onium chloroiodates and molecular iodine and its addition products, e.g. with polyvinylpyrrolidone, with polyoxyalkylenes and their derivatives or with quaternary ammonium compounds.

Suitable inorganic iodides are for instance, calcium iodide, ammonium iodide, lithium iodide, magnesium iodide, potassium iodide, sodium iodide, barium iodide, cadmium iodide, and zinc iodide.

Suitable organic iodides are for instance the iodides having the following structural formulae:

tetramethylammonium iodide,

'(H CCH N+]I-3I tetraethylammonium iodide-3-diiodine,

1,1,l-dodecyldimethylhydrazinium( l iodide,

O H (IJH:

1-methyl-8-hydroxy-quinolinium iodide,

1methyl-2-iodo-quinolinium iodide,

OH H1O CH:

1,2,3,4 tetrahydro 8 hydroxy 1,1 dimethyl-quinolinium iodide,

H30 CH:

S,S'-bis-(dimethyl) hexamethylene 1,6 disulphonium iodide,

3,S-dimorpholino-dithiolium iodide, and

diphenyl-iodonium iodide.

Organic compounds with labile iodine atom which have proved to be suitable for use according to the invention are, e.g., mono-iodoacetic acid and 4-iod0-butane sulphonic acid potassium salt.

Finally, as regards the onium chloroiodates there may be referred to Belgian patent specification No. 515,895; examples of suitable onium chloroiodates are among others more particularly:

trimethyl(o methoxycarbonyl) anilinium dichloroiodate, and

benzyltriphenyl-phosphonium dichloroiodate.

Incorporating a compound releasing bromide ions in an aqueous medium, this compound being other than silver bromide, within the silver halide emulsion layer subsequent to precipitation of the silver halide generally still further increases the maximum density of the direct positive image. As will be shown in Example 12 hereinafter, the favorable etfect obtained by incorporating a compound releasing iodide ions and a compound releasing bromide ions in the silver halide emulsion layer before processing cannot be obtained if these compounds are present in the developer itself.

The compound releasing bromide ions is mostly incorporated within the silver halide emulsion layer at the same concentration range given above for the compounds releasing iodide ions.

Suitable compounds releasing bromide ions are among other inorganic and organic bromides such as ammonium bromide, lithium bromide, sodium bromide, potassium bromide, magnesium bromide, calcium bromide, barium bromide, cadmium bromide and zinc bromide as inorganic bromides and tetraethyl ammonium bromide and ethyl pyridinium bromide as organic bromides. Organic compounds with labile bromine atom such as monobromoacetic acid are also suitable for being used in the present process as compounds setting free bromide ions.

In a first step for the preparation of the direct positive image the light-sensitive material is image-wise exposed to the original to be reproduced. This exposure can be either a high-intensity exposure such as flash exposure or a normal-intensity exposure such as daylight exposure as well as a low-intensity exposure such as an exposure by means of a printer or an exposure of still lower intensity, the exposure-time being dependent on the sensitivity of the light-sensitive material for the direct positive image formation which sensitivity according to the present invention may be a camera speed as Well as a lower sensitivity.

In a further step of the direct positive image formation according to the present invention the image-wise exposed light-sensitive material is treated with an aqueous energetic surface developing liquid. This treatment may be carried out according to any technique known in the art, e.g. by soaking or by wetting only one side of the lightsensitive material, e.g. by means of a lick roller, by spreading a paste, e.g. contained in a pod, or by spraying.

The developing liquid, also called developer, of use in the process of the invention must be a surface developer, i.e. a developer that contains no or at least no effective amount of solvents for silver halide. In this case by solvents for silver halide there are meant particularly strong solvents for silver halide such as water-soluble thiocyanates, thiosulphates, ammonia, etc. Indeed, compounds that may be considered to be but very weak solvents for silver halide, such as sodium sulphite in case the silver halide is silver bromide, silver chlorobromide, or silver bromoiodide, may be present in the developer.

Further, the developer has to be an energetic developer. The high energy required can be realized by taking, e.g. one or more of the following measures:

using a developer with relatively high pH value, working at an elevated temperature, using a developer wherein the concentration of the various compounds present is relatively high, and incorporating developing activators such as cyclohexylamine, polyethylene, glycols and derivatives in the developer.

The choice of the developing substances too determines the energy of the developer. Examples of suitable developing substances are among others the combination of 1- phenyl-3-pyrazolidinone with hydroquinone and the combination of monomethyl-p-aminophenol sulphate with hydroquinone. In general there may be said that the more energetic the developer is the better the result obtained will be.

For carrying out the method of the invention the developing agents may also be incorporated partially or even completely into the light-sensitive material. This incorporation may be accomplished during the preparation state of the material or later on by means of a processing liquid, with which the light-sensitive material is wetted prior to the development of the direct positive image. In doing so the energetic surface developer can be reduced to a mere alkaline liquid substantially free from developing agents. Such an alkaline aqueous liquid, often called activator, offers the advantage of having a longer activity, i.e. of being less rapidly exhausted.

The developer (occasionally activator) may be supplied in an amount, which suflices for the treatment of exactly one foil of light-sensitive material. A bath of this type offers the advantage that ageing and contamination of the bath composition are avoided.

While the image-wise exposed light-sensitive layer is present in the developer or is moistened therewith in some way the said light-sensitive material is overall and generally uniformly exposed to actinic light of low intensity, i.e. of an intensity which is lower than a normal daylight intensity. This exposure may begin simultaneously with the treatment of the light-sensitive material with developer but occurs favourably a little later for instance from 5 to 30 seconds later. The duration of the secondary or overall exposure is not very critical and may amount from seconds to many minutes according to the nature of the light-sensitive material, the composition and the temperature of the developer, the intensity of the lightsource employed, etc. In many cases the overall exposure lasts till the direct positive image is completely developed. This offers the additional advantage of being able to follow the development and stop it when the direct positive image formation is thought to have reached its optimum value. The density of the direct positive image can be made more intense or reduced locally during the second or overall exposure by the well known technique of burning in and dodging. In general the total development of the direct positive image does not take much time and in most cases may be carried out in from 1 to 5 minutes. The uniformity of the said overall exposure to actinic light of low intensity is mostly achieved by interposing between the light-source and the light-sensitive material to be exposed a light-diffusing member that transmits at least part of the light rays of the spectral region for which the lightsensitive material is sensitive. The required low intensity of the light rays, which reach the light-sensitive material can be obtained by adapting the light-source and/or the light-diffusing member.

As already stated above, the direct positive images obtained according to the present invention show very high maximum densities and have sufiiciently low minimum densities.

This minimum density can still be lowered by incorporating a fog-inhibiting compound that inhibits fogging during the development stage and that belongs to the class of the heterocyclic thione compounds, within the light-sensitive silver halide emulsion layer. According to a preferred embodiment the fog-inhibiting compound is incorporated within the silver halide emulsion layer by adding it to the liquid silver halide emulsion, generally just before coating. However, the fog-inhibiting compound may also be incorporated within the emulsion layer by difiusion from or through another layer, which is in water-permeable relationship with the emulsion layer. This diffusion occurs automatically and immediately when the silver halide emulsion is applied to a water-permeable layer containing the fog-inhibiting compound or when an aqueous coating liquid containing said compound is applied to the silver halide emulsion layer or to a water-permeable layer present on top of the emulsion layer. In any case the foginhibiting compound has to be present in the silver halide emulsion layer before the aqueous energetic surface developer is applied to the image-wise exposed light-sensitive material. Adding the fog-inhibiting compound to the developing liquid instead of incorporating it within the silver halide emulsion layer as described above results in a considerable decrease of the maximum density of the direct positive image as will be shown in Example 13 hereinafter.

The fog-inhibiting compounds give rise to a lowering of the minimum density of the direct positive image already when present in only small amounts. They are very eflicient when applied in the common fog-inhibiting amounts, i.e. in amounts of up to 0.1 mole per moles of silver halide. The best results, i.e. the most clear minimum densities are however attained when using the fog-inhibiting compounds in amounts larger than the common fog-inhibiting amounts, i.e. in amounts of at least 0.1 mole per 100 moles of silver halide. These large amounts of fog-inhibiting compound normally are not applied to the light-sensitive materials since with such amounts the light-sensitive material would be too strongly desensitized. What is surprising now is that in the process of the present invention, these large amounts of fog-inhibiting compound do not lower the direct positive image sensitivity of the lightsensitive material but in most cases even enhance it and also do not prevent the formation of a direct positive image with good maximum density.

Specific examples of suitable fog-inhibiting compounds belonging to the class of the heterocyclic thione compounds are:

4-phenyl-A -thiazoline-2-thione,

4,5-diphenyl-A -thiazoline-Z-thione,

benzothiazoline-2-thione,

4-phenyl-A -oxazoline-2-thione,

4,5-diphenyl-A -oxazoline-Z-thione,

4-phenyl-A -imidazoline-2-thione,

4,5-diphenyl-A -imidazoline-2-thione,

l-methyl-benzimidazoline-Z-thione,

1-phenyl-imidazolidine-2-thione,

3-phenyl-A -1,2,4-thiadiazoline-S-thione,

4-phenyl-A l ,2,4-triazoline-5-thione,

3-phenyl-A -1,2,4-triazoline-5-thione,

l-methyl-2-tetrazoline-S-thione,

1-phenyl-2-tetrazoline-5-thione,

1-( l-naphthyl -2-tetrazoline-5'thione,

1- Z-naphthyl) -2-tetrazoline-5-thione,

1- (9-anthryl -2-tetrazoline-5-thione,

1- 3,4-dichlorophenyl -2-tetrazoline-5-thione,

1- o-methoxy-phenyl -2-tetrazoline-5-thione,

1- (o-biphenyl) -2-tetrazoline-5-thione,

1- (p-biphenyl) -2-tetrazoline-5-thione,

1- Z-naphthyl) -4,4,6-.trimethyl-l ,2,3,4-tetrahydropyrimidine-Z-thione The best results are obtained with five-membered heterocyclic thione compounds carrying an aryl substituent or a fused-on aromatic group. Most of the specific examples of suitable heterocyclic thione compounds listed hereinbefore belong to this subclass.

It will be understood that any reference in the description and in the claims to fog-inhibiting compounds of the class of the heterocyclic thione compounds is intended to refer also to the tautomeric structures of said compounds and to salts of said compounds or occasionally of said tautomeric structures. Mixtures of two or more fog-inhibiting compounds may also be employed.

In the method according to the invention the maximum or minimum density of the direct positive image can still be improved and the characteristics of the light-sensitive material can be altered by using all kinds of ingredients which are generally known in the art of emul sion preparation and some of which will be set out specifically hereinafter.

Most of these ingredients are preferably incorporated into the light-sensitive material itself, in effective contact with the silver halide emulsion layer and favourably in the latter layer itself. Many of these ingredients, however, may be incorporated into the energetic surface developer with the same favourable result. Combinations of two or more of said ingredients, of course, may also be used.

Among the said ingredients may be mentioned some binders for at least partially replacing gelatin as a binder for the silver halide grains and referred to already above in the description, further optical sensitizers and moistening agents such as polyalkylene glycols; i.a. polyethyleneglycols, sulphonated fatty acids, saponine and the like. The presence of optical sensitizers and of polyalkylene glycols in the silver halide emulsion layer in most cases has a favourable effect on the maximum density of the direct positive image. Still further ingredients are mentioned later on in the description.

The process according to the invention is not restricted to black and white photography; it may also be applied for the preparation of direct positive colour images by carrying out the development in an energetic colour developer in the presence of a colour coupler and by bleaching away the silver developed. The light-sensitive material, of course, may be a multilayer colour material each layer of which absorbs a particular part of the spectrum and for which layers appropriate colour couplers are provided. In addition to their developing or coupling action many colour developing compounds and colour couplers exert an advantageous effect on the mechanism of the direct positive image production itself in that they contribute to a higher maximum and/or a lower minimum density.

According to the method of the invention a negative image is formed in the silver halide emulsion layer in addition to the direct positive image, just as is the case with the Sabattier effect. In the latter case, however, the positive and negative image overlap with each other whereas according to our invention there is a clear separation between the direct positive and the negative image that forms in a lower sensitivity range. This separation is clearest when the image-wise exposure is a high-intensity exposure, e.g. a flash exposure and is less clear accordingly as the intensity of the image-wise exposure is lower. Nevertheless, excellent results can still be attained with a normal daylight exposure and even with an exposure of still lower intensity, for instance an exposure by means of a printer.

In the light-sensitive material for use in the method of the invention a negative image in the low sensitivity range can also be obtained by developing this material after image-wise exposurein a surface developer without subjecting it to a second or overall exposure.

By developing the light-sensitive material for use ac cording to the present invention after image-wise exposure in an internal developer a negative image is obtained that lies in a sensitivity range only slightly lower than that of the direct positive image obtained with a same material according to the process of the present invention.

Thus the direct positive image obtained by proceeding according to the method of the invention lies in the highest sensitivity range.

The following are examples illustrating the process of the present invention.

EXAMPLE 1 A gelatino silver bromide emulsion that forms a latent image predominantly in the inner part of the silver halide grains is prepared through conversion of a silver chloride emulsion by very slowly adding to said emulsion whilst stirring a 35% aqueous potassium bromide solution in an amount that is 70% higher than the amount theoretically necessary for converting all the silver chloride. The emulsion is then kept for 1 hour at 60 C. After the emulsion has been chilled and allowed to gel for 6 hours the emulsion is noodled. The noodles are washed for 1 hour with water (10 litres of water per minute). By heating the washed noodles a liquid silver bromide emulsion is obtained containing 50 g. of gelatin and 0.4 mole of silver bromide per kg.

To this liquid emulsion potassium iodide is added as follows: to a first part of the emulsion 6 cc. of a 5% aqueous potassium iodide solution are added per kg., to a second part 10 cc. of such solution are added per kg. and to a third part 20 cc. of such solution are added per kg.

Each of the light-sensitive emulsions obtained is coated, practically immediately after the addition of the potassium iodide solution, on a baryta-coated paper support of g./sq. m. in such a way that an amount of silver halide equivalent to 4 g. of silver nitrate is present per sq. m. of light-sensitive material.

The emulsion layer of each of the materials is overcoated with a conventional hardened gelatin antistress layer.

The three silver halide materials obtained are further treated in a completely identical way. Each material is image-wise exposed for l/ioyoor) sec. through a stepwedge by means of a flash exposure of 130 lux sec. Then each material is developed for 4 min. at 20 C. in a surface developer of the following composition:

Hydroquinone g 15 1-phenyl-3-pyrazolidinone g 1 Sodium carbonate (anhydrous) g 30 Sodium sulphite (anhydrous) g 40 Water up to cc 1000 Sodium hydroxide till pH 11.

30 see. after the development has started the lightsensitive material is overall exposed for the remaining time of the development through a grey filter having a density of 2.7 means of a IS-Watt lamp placed at a distance of 70 cm. from the light-sensitive material. Each of the light-sensitive materials is then fixed, rinsed and dried in the conventional way.

A direct positive image of the original is obtained in each of the three materials. The three images all have a low minimum density and a high maximum density which is the higher the more potassium iodide has been incorporated into the light-sensitive material. As compared with a direct positive image obtained in the same manner but with a light-sensitive material containing no potassium iodide, the direct positive images obtained according to the method of the invention as described in this example, show a much higher maximum density and practically the same minimum density.

EXAMPLE 2 A gelatino silver bromide emulsion that forms a latent image predominantly in the inner part of the silver halide grains is prepared through conversion of a silver chlorobromide emulsion (10 mole percent of bromide) by very slowly adding to said emulsion whilst stirring a 35% aqueous potassium bromide solution in an amount that is 90% higher than the amount theoretically necessary for converting all the silver chloride. The emulsion is then kept for 1 hour at 60 C. After the emulsion has been chilled and allowed to gel for 6 hours the emulsion is noodled. The noodles are washed for 1 hour with water (10 litres of water per minute). By heating the Washed noodles a liquid silver bromide emulsion is obtained containing 50 g. of gelatin and 0.4 mole of silver bromide per kg.

To this liquid emulsion an appropriate colour coupler for yellow is added in a concentration as commonly applied as well as potassium iodide in the following amounts: to a first part of emulsion 2.5 cc. of percent aqueous potassium iodide solution are added per kg of emulsion and to a second part 5 cc. of such solution are added per kg. of emulsion.

Practically immediately thereafter each of the lightsensitive emulsions is coated on a 'baryta-coated paper support of 130 g./sq. m. in such a way that an amount of silver halide equivalent to 4 g. of silver nitrate is present per sq. m. of light-sensitive material.

The emulsion layer of each of the materials is overcoated with a conventional hardened gelatin antistress layer.

The two silver halide materials obtained are further treated in a completely identical way. Each material is image-wise exposed for sec. through a step-wedge by means of a flash exposure of 130 lux sec. Then each material is developed for 2 min. at 26 C. in a developer of the following composition:

N-ethyl N hydroxyethyl p phenylenediamine hydrochloride g Anhydrous sodium sulphite g 4 Anhydrous sodium carbonate g 50 Hydroxylamine hydrochloride g 3 Water up to cc 1000 The said development occurs for the first 30 sec. in the dark. During the remaining time of the development the materials are overall exposed through a grey filter with a density of 2.1 by means of a l5-watt lamp placed at a distance of 70 cm. from the light-sensitive material.

After the development, each material is treated in a stop bath, bleached, fixed and rinsed with water as is the case with conventional photographic colour material.

A yellow positive image of the step-wedge is obtained in each of the two materials. These two images have a good minimum density and a high maximum density which is the higher the more potassium iodide has been incorporated into the light-sensitive material. As compared with a direct positive image obtained in the same manner but with a light-sensitive material in which no potassium iodide had been incorporated, the direct positive images obtained according to the method of the invention as described in this example show a much higher maximum density and practically the same minimum density.

EXAMPLE 3 Example 2 is repeated as far as the light-sensitive material, in which the highest amount of potassium iodide has been incorporated is concerned. However, immediately after havingincorporated said potassium iodide and before coating a 5% solution in ethanol of l-(o-methoxyphenyl)-2-tetrazoline-5-thione is also added to the silver halide emulsion in an amount of 2.5 cc. per mole silver bromide, which is an amount of fog-inhibiting compound as commonly employed.

The direct positive image obtained has a minimum density lower than that obtained in the corresponding material of Example 2 where no fog-inhibiting compound has been incorporated into the light-sensitive material- Its maximum density has practically not been changed as compared with the material comprising no fog-inhibiting compound.

12 EXAMPLE 4 Test A Example 1 is repeated but now the liquid emulsion is not divided into different parts to which potassium iodide is added in different amounts. In this example 10 cc. of a 5% aqueous solution of potassium iodide and 10 cc. of a 5% solution in ethanol of 1-(o-methoxyphenyl)-2- tetrazoline-S-thione are added to the undivided liquid silver halide emulsion per kg. The maximum and minimum densities of the direct positive image obtained are given in Table I (Test A).

Test B Test A is repeated with the difference however that at the moment of the addition of the potassium iodide and fog-inhibiting compound 5 cc. of a 3 N aqueous solution of cadmium bromide are also added per kg. of liquid silver halide emulsion. The maximum and minimum density of the direct positive image obtained are given in Table I (Test B).

Test C Test B is repeated with the difference however, that at the moment of the addition of the potassium iodide, foginhibiting compound and cadmium bromide 50' cc. of a 20% aqueous solution of poly(N-vinyl pyrrolidone) (mole weight: about 40.000) are also added per kg. of liquid silver halide emulsion. The maximum and minimum density of the direct positive image obtained are given in Table I (Test C).

Test D Test C is repeated with the difierence however that no cadmium bromide is added and that instead of 10 cc. of a 5% aqueous solution of potassium iodide, 5 cc. of a 50% aqueous solution of potassium iodide are added per kg. of emulsion. The maximum and minimum density of the direct positive image obtained are given in Table I (Test D).

TABLE I Maximum Minimum density density Test:

A 0.61 0.10 B 0.69 0.10 C 1.25 0.17 D 1.39 0.16

EXAMPLE 5 Test E Test F Test E is repeated with the difference however that at the moment of the addition of the potassium iodide and other substances mentioned also 5 cc. of a 3 N aqueous solution of cadmium bromide are added per kg. of liquid silver halide emulsion. The maximum and minimum density of the direct positive image obtained are given in Table II (Test F).

13 Test G Test H Test F is repeated with the difference however that at the moment of the addition of the potassium iodide and other substances mentioned also 50 cc. of a 20% aqueous solution of poly(N-vinyl pyrrolidone) (mole weight about 40.000) are added per kg. of liquid silver halide emulsion. The maximum and minimum density of the direct positive image obtained are given in Table II (Test H).

TABLE II Maximum Minimum density density EXAMPLE 6 A photographic multilayer colour material is prepared by coating a baryta-coated paper supported of 130 g./ sq. 111. according to the generally known procedure successively with: a panchromatically sensitized silver halide emulsion layer, an intermediate gelatin layer, an orthochromatically sensitized silver halide emulsion layer, once again an intermediate gelatin layer, a yellow filter layer, a blue sensitive silver halide emulsion layer and a conventional hardened gelatin antistress layer. The various emulsion layers are coated pro rata of /1000 to 5/100 mole of silver halide per sq. m. of material according to the nature of the emulsion, the colour coupler used, the gradation desired, etc.

For the preparation of the light-sensitive emulsion layers a gelatino silver bromide emulsion that forms a latent image predominantly in the inner part of the silver halide grains is prepared through conversion of a silver chloride emulsion by very slowly adding to said emulsion whilst stirring a 35% aqueous potassium bromide solution in an amount that is 70% higher than the amount theoretically necessary for converting all the silver chloride. The emulsion is then kept for 1 hour at 60 C. After the emulsion has been chilled and allowed to gel for 6 hours the emulsion is noodled. The noodles are washed for 1 hour with water litres of water per minute). By heating the washed noodles a liquid silver bromide emulsion is obtained containing 50 g. of gelatin and 0.4 mole of silver bromide per kg.

The coating composition of the panchromatic emulsion layer is obtained by adding to the liquid silver bromide emulsion obtained on heating the washed noodles, in addition to the usual hardening agents and coating aids, an appropriate panchromatic sensitizer and an appropriate colour coupler for cyan both in a concentration as commonly applied as well as the following solutions the amounts of which are given per mole of silver halide:

60 cc. of a 5% solution in ethanol of l-(o-methoxyphenyl)-2-tetrazoline-5-thione 30 cc. of a 1% solution in isopropanol of Z-mercaptoethylcarbanilate,

30 cc. of a 5% aqueous solution of potassium iodide, and

30 cc. of 3 N aqueous solution of cadmium bromide.

The coating composition of the orthochromatic emulsion layer is obtained by adding to the liquid silver bromide emulsion obtained on heating the Washed noodles, in addition to the usual hardening agents and coating aids, an appropriate orthochromatic sensitizer and an appropriate colour coupler for magenta both in a concentration as commonly applied as well as the following solutions, the amounts of which are given per mole of silver halide:

cc. of a 5% solution in ethanol of l-(o-methoxyphenyl)-2-tetrazoline-5-thione,

45 cc. of a 1% solution in isopropanol of 2-mercaptoethyl-carbanilate,

45 cc. of a 5% aqueous solution of potassium iodide,

45 cc. of a 3 N aqueous solution of cadmium bromide,

and

200 cc. of a polyethylacrylate latex (40 g. per cc.

latex).

The coating composition of the blue-sensitive emulsion layer is obtained by adding to the liquid silver bromide emulsion obtained on heating the Washed noodles, in addition to the usual hardening agents and coating aids, an appropriate blue-sensitizer and an appropriate colour coupler for yellow both in a concentration as commonly applied as well as the following solutions, the amounts of which are given per mole of silver halide:

30 cc. of a 5% solution in ethanol of l-(o-methoxyphenyl)-2-tetrazoline-5-thione,

5 cc. of a 1% solution in isopropanol of Z-mercaptoethyl carbanilate, and

10 cc. of a 3 N aqueous solution of cadmium bromide.

The photographic multilayer colour material obtained is exposed to a coloured continuous tone original and then developed for 3 min. at 26 C. in a developer of the following composition:

N-ethyl N hydroxyethyl-p-phenylenediamine hydrochloride g 10 Anhydrous sodium sulphite g 4 Anhydrous sodium carbonate g 50 Hydroxylamine hydrochloride g 3 Water up to cc 1000 The said development occurs for the first 30 sec. in the dark. During the remaining time of the development the material is overall exposed to light of low intensity but actinic for all three light-sensitive layers.

After the development, the material is treated in a stop bath, bleached, fixed and rinsed with water as in the case with conventional photographic colour material. A direct positive image of good quality is obtained.

The grey rendering can be influenced by changes in the emulsion composition of the various emulsion layers but also by varying the light of the image-wise exposure and/ or of the low-intensity overall exposure by means of filters.

EXAMPLE 7 A gelatino silver bromide emulsion that forms a latent image predominantly in the inner part of the silver halide grains is prepared through conversion of a silver chloride emulsion by very slowly adding to said emulsion whilst stirring a 35% aqueous potassium bromide solution in an amount that is considerably higher than the amount theoretically necessary for converting all the silver chloride. The emulsion is then kept for a certain time at 60 C.

After the emulsion has been chilled and allowed to gel for 6 hours the emulsion is noodled. The noodles are washed for 1 hour with water (10 litres of water per minute).

By heating the washed noodles, again a liquid silver halide emulsion is obtained (ratio of gelatin/silver nitrate g. per 100 cc. of solution) 4 Aqueous solution of potassium iodide (5 g. per 100 cc. of solution) 6 Solution of 2-mercaptoethyl-carbanilate in isopropanol (1 g. per 100 cc. of solution) 5 4% by weight aqueous solution of formaldehyde 5 5% by volume solution of in a mixture of ethanol and water (50:50) Aqueous solution of R-CH SO Na wherein R represents an alkyl group comprising from 14 to 18 carbon atoms (5 g. per 100 cc. of water) 15 This light-sensitive emulsion is coated practically immediately after the addition of the above solution on a baryta coated paper support of 130 g./sq. m. in such a way that an amount of silver halide equivalent to 4 g. of silver nitrate is present per sq. m. of light-sensitive material.

The emulsion layer is overcoated with a conventional hardened gelatin antistress layer.

The light-sensitive silver halide material obtained is exposed for 1/ 10,000 sec. through a step-wedge by means of a flash exposure of 130 lux sec.

A first strip of this image-wise exposed light-sensitive material is developed for 2 min. at 20 C. in a surface developer of the following composition:

Sodium hydroxide till pH 11.

The strip is then fixed, rinsed and dried in the conventional way. The densities obtained at the various log exposure (log E) values are represented by curve I of the accompanying drawing.

A second strip of the above image-wise exposed lightsensitive material is treated in the same way as the first strip with the difference, however, that 30 see. after the development has started the light-sensitive material is overall exposed for the remaining time of the development through a grey filter having a density of 2.4 by means of a 15-watt lamp placed at a distance of 45 cm. from the light-sensitive material. The densities obtained at thelvarious log exposure (log E) values are represented by curve II of the accompanying drawing.

A third strip of the above image-wise exposed lightsensitive material is treated in the same way as the first strip with the difference, however, that the developer comprises in addition to the above ingredients 5 g. of anhydrous sodium thiosulphate per litre so that it is an internal developer. The densities obtained at the various log exposure (log E) values are represented by curve III of the accompanying drawing.

The above curves show clearly that when an imagewise exposed light-sensitive material suitable for use according to the process of the invention is developed in a surface developer a negative image of the original having a very low density is obtained (curve I);

is developed in an internal developer a negative image of the original is obtained having a higher density than that obtained on development in a surface developer and forming in a higher sensitivity range than that wherein a negative image is obtained on development in a surface developer (curve III);

is treated according to the method of the invention a direct positive image with suitable maximum and lox minimum density is obtained in a sensitivity range that is even higher than that wherein a negative image is obtained on development in an internal developer (curve II).

EXAMPLE 8 A light-sensitive silver halide emulsion is prepared in an analogous way as described in Example 7 with the difference however, that now a smaller amount of potassium bromide is used in order to convert the silver chlo ride emulsion into a silver bromide emulsion, and that just before coating, the following series of solutions is added to the emulsion instead of those listed in Example 7:

Solution in ethanol of the following sensitizer (1 g. per

1000 cc. of solution) s OCH=CHCH=C/ \CH Aqueous solution of cadmium bromide (40 g. per 100 cc. of solution) 5 Solution of 1-(o-methoxyphenyD-2-tetrazoline-5-thione in isopropanol (5 g. per 100 cc. of solution) 8 Solution of Z-mercaptoethyl-carbanilate in isopropanol (1 g. per 100 cc. of solution) 5 Aqueous solution of potassium iodide (5 g. per 100 cc. of solution) 6 4% by weight aqueous solution of formaldehyde 5 5% volume solution of in a mixture of ethanol and Water (50/50) Aqueous solution of RCH SO Na wherein R represents an alkyl group comprising from 14 to 18 carbon atoms (5 g. per 100 cc. of water) Aqueous dispersion of polyethylacrylate (40 g. per 100 cc. of dispersion) containing as emulsifying agent the sodium salt of oleyl methyl tauride in a concentration of 5% by weight relative to the total amount of polyethylacrylate The light-sensitive silver halide emulsion is coated practically immediately after the addition of the above solutions on a cellulose triacetate film support provided with a gelatin subbing layer in such a way that an amount of silver halide equivalent to 8 g. of silver nitrate is present per sq. m. of light-sensitive material.

The light-sensitive material is exposed in a camera, the exposure being as for material having a film speed of ASA. The material is then developed at 25 C. in a surface developer as described in Example 7. After having been developed for 30 sec. in the dark the light-sensitive material is developed for another 3 min. 30 sec. while being overall exposed at both sides through a grey filter having a density of 2.4 by means of a 15-watt lamp placed at a distance of 45 cm. from the light-sensitive material. After fixing, rinsing and drying the developed light-sensitive material in the conventional way a direct positive transparency of excellent sharpness and gradation is obtained.

EXAMPLE 9 A light-sensitive silver halide material is prepared in an analogous way as described in Example 7 with the only difference that in addition to the potassium bromide a lit- 1 7 tle potassium iodide is used in the conversion of the silver chloride emulsion until finally a silver bromoiodide emulsion comprising only a minor amount of silver iodide is obtained.

By means of a camera with a pentaprism system a portrait is made on the light-sensitive silver halide material. After exposure, the material is developed, fixed, rinsed and dried in the same way as the second strip of the light-sensitive material of Example 7.

In this way a direct positive continuous tone image of excellent quality is obtained in a very short time.

EXAMPLE A light-sensitive silver halide material is prepared as described in Example 7 with the difference, however, the 500 g. of an aqueous solution of 25 g. of gelatin and 17 g. of the following colour coupler are added to the gelatino silver halide emulsion just before the addition of the final series of solutions.

The light-sensitive material obtained is image-wise exposed with daylight for 1/20 sec. through a step-wedge.

After the image-wise exposure the light-sensitive material is developed at 28 C. in a developer of the following composition:

N,N-diethyl p phenylenediamine hydrochloride g 3 Sodium sulphite (anhydrous) g 4 Sodium carbonate (anhydrous) g 50 Hydroxylamine hydrochloride g 3 Water up to cc 1000 First the light-sensitive material is developed for 60 sec. in the dark and then for 120 sec. while being overall exposed through a grey filter with a density of 0.9 by means of a -watt lamp placed at a distance of 45 cm. from the light-sensitive material.

After the development, the material is treated in a stop bath, bleached, fixed and rinsed with water as is the case with conventional photographic colour material. A magenta coloured direct positive image of the step-wedge is obtained showing dark areas with good density and nearly pure white areas.

EXAMPLE 11 A gelatino silver bromide emulsion that forms a latent image predominantly in the inner part of the silver halide grains is prepared through conversion of a silver chloride emulsion by very slowly adding to said emulsion whilst stirring a 35% aqueous potassium bromide solution in an amount that is 70% higher than the amount theoretically necessary for converting all the silver chloride. The emulsion is then kept for 1 hour at 60 C. After the emulsion has been chilled and allowed to gel for 6 hours the emulsion is noodled. The noodles are washed for 1 hour with water (10 litres of water per minute). By heating the washed noodles a liquid silver bromide emulsion is obtained containing 50 g. of gelatin and 0.8 mole of silver bromide per kg.

To this liquid emulsion are added per kg. of emulsion 7.5 cc. of a solution of 3.8 g. of molecular iodine in 100 cc. of ethanol.

The light-sensitive emulsion obtained is coated, practically immediately after the addition of the molecular iodine, on a baryta-coated paper support of 130 g./sq. m. in such a way that an amount of silver halide equivalent to 4 g. of silver nitrate is present per sq. m. of light-sensitive material.

The emulsion layer is over-coated with a conventional hardened gelatin antistress layer.

The silver halide material is image-wise exposed for 1/ 10,000 sec. through a step-wedge by means of a flash exposure of lux sec. Then the material is developed for 4 min. at 20 C. in a surface developer of the following composition:

Hydroquinone g 15 1-phenyl-3-pyrazolidinone g 1 Anhydrous sodium carbonate g 30 Anhydrous sodium sulphite g 40 Water up to cc 1000 Sodium hydroxide till pH 11.

30 sec. after the development has started the light-sensitive material is overall exposed for the remaining time of the development through a grey filter having a density of 2.7 by means of a 15-Watt lamp placed at a distance of 70 cm. from the light-sensitive material. The lightsensitive material is then fixed, rinsed and dried in the conventional way.

A direct positive image of the original is obtained having a low minimum density and a high maximumv density. As compared with a direct positive image obtained in the same manner but with a light-sensitive material to which no iodine has been added the direct positive images obtained according to the method of the invention as described in this example, shows a much higher maximum density and practically the same minimum density.

EXAMPLE 12 A gelatino silver bromide emulsion that forms a latent image predominantly in the inner part of the silver halide grains was prepared through conversion of a silver chloride emulsion by very slowly adding to said emulsion whilst stirring a 35 aqueous potassium bromide solution in an amount that was 70% higher than the amount theoretically necessary for converting all the silver chloride. The emulsion was then kept for 1 hour at 60 C. After the emulsion had been chilled and allowed to gel for 6 hours it was noodled. The noodles were washed for 1 hour with water (10 litres of water per minute). By heating the washed noodles a liquid silver bromide emulsion Was obtained containing 50 g. of gelatin and 0.4 mole of silver bromide per kg.

Per kg. of the liquid emulsion thus obtained the following solutions were added:

Cc. 35 aqueous solution of potassium bromide 25 5% aqueous solution of potassium iodide l2 5% solution of l-(o-methoxy-phenyl)-2-tetrazoline-5- thione in ethanol 20 After the addition of an optical sensitizer and some other usual coating aids, the silver halide emulsion was applied to a baryta-coated paper support provided with a thin layer of cellulose nitrate. It was applied in such a way that an amount of silver halide equivalent to 5.5 g. of silver nitrate was present per sq. m. of the lightsensitive material.

Finally the emulsion layer was provided with a conventional hardened gelatin antistress layer.

The light-sensitive material was image-wise exposed for 1 second through a step wedge having a wedge constant of 0.1 'by means of a low intensity lamp. It Was then developed for 65 seconds at 30 C. in a developer having the following composition:

Hydroquinone g 15 l-phenyl3-pyrazolidinone g 1 Sodium sulphite (anh.) g 70 Sodium carbonate (anh.) g 30 Trisodium salt of ethylenediamine tetra-acetic acid g 1 40% aqueous sodium hydroxide cc 16 Water up to cc 1000 25 seconds after the development had been started the light-sensitive material was overall exposed for the re maining time of the development through a grey filter having a density of 2.7 by means of a 15-Watt lamp placed at a distance of 70 cm. from the light-sensitive material. The light-sensitive material was then fixed, rinsed and dried in the conventional way. The direct positive image obtained shows a good gradation, a low minimum density (0.32) and a high maximum density (1.66).

A second direct positive image was prepared in an analogous manner with the difiFerence, however, that the potassium bromide and the potassium iodide were omitted from the light-sensitive material and instead thereof were incorporated into the developer by adding 20 cc. of a 10% aqueous solution of potassium bromide and 10 cc. of a 1% aqueous solution of potassium iodide per litre of developer. The amounts of potassium bromide and potassium iodide in the developer had been chosen in such a way that the most favourable result was obtained. The direct positive image thus obtained shows a low minimum density (0.30) indeed, but also shows too low a gradation and maximum density (0.95).

EXAMPLE 13 A direct positive image was prepared as the first direct positive image described in Example 12. This direct positive image shows a good gradation, a low minimum density (0.32) and a high maximum density (1.66).

A second direct positive image was prepared in an analogous manner with the dilference, however, that 1- (o-methoxyphenyl)-2-tetrazoline 5 thione was omitted from the light-sensitive material and instead thereof was incorporated into the developer by adding 25 cc. of a 5% solution in ethanol per litre of developer. The amount of fog-inhibiting compound in the developer had been chosen in such a way that the most favorable result was obtained. The direct positive image thus obtained shows a somewhat lower minimum density (0.24) indeed, but also shows a considerably lower maximum density (1.25) and a lower gradation.

EXAMPLE 14 A series of direct positive images was prepared in an analogous manner as the first direct positive image described in Example 12. However, instead of adding 20 cc. of a 5% solution of 1-(o-methoxy-phenyl)-2-tetrazoline-S-thione in ethanol per kg. of the liquid silver bromide emulsion, the fog-inhibiting compounds listed in the table hereinafter were added to the liquid emulsion in the given amounts.

The following results were obtained:

TABLE Amountof fog-1nhibiting compound in mg. per

kg. liquid Maxr- Minisilver mum mum bromide dendeu- A den- Fog-inhibiting compound emulsion sity sity sity 1-(o-methoxy-phenyl)-2tetraz oline-5-thioue 750 1. 75 0. 31 1. 44 4-phenyl-A -thiazoline-2-thione-..- 693 1. 55 0. 26 1. 29 4, 5dipheuyl-A -thiazoline-2- thione 968 1. 0. 22 0. 78 Benzothiazoline-Zthione. 600 1. 44 0. 23 1. 21 4-phenyl-A -oxazoline-2-th no 636 1. 22 0. 22 1. 00 4, E-diphenyl-N-oxazoline-Z- thione 910 l. 50 0. 23 1. 27 4-phenyl-N-imldazoline-2-thione- 634 1. 0. 23 0. 82 4, fi-diphenyl-A -imidazoliue-Z- one 908 0. 94. 0. 21 0. 73 l-methyl-benzimidazo1ine-2- thione 509 1. 48 0. 25 1. 23 1-pheny1-imidazolidine-2-thione. 640 1. 09 0. 29 0. 80 3-phenyl-A -1, 2, 4-thiadiazolinefi-thione 698 1. 11 0. 33 0. 78

We claim:

1. In a method of producing a photographic direct positive image that comprises the steps of:

imagewise exposing a light-sensitive material, which comprises a layer of an unfogged silver halide emulsion of the type that forms a latent image predominantly in the inner part of the silver halide grains,

treating the light-sensitive material thus exposed with an aqueous energetic surface developing liquid, and overall exposing the light-sensitive material during the treatment stage to actinic light having an intensity lower than a normal daylight intensity to produce a direct positive image in said emulsion layer,

the improvement which comprises incorporating within said emulsion layer subsequent to precipitation of the silver halide of said emulsion a compound other than silver iodide releasing iodide ions in an aqueous medium in an amount suflicient to effectively increase the density and contrast of the direct positive image in said emulsion layer.

2. A method according to claim 1, wherein a foginhibiting compound, which inhibits fogging during the development stage and which belongs to the class of the heterocyclic thione compounds is incorporated within said emulsion layer.

3. A method according to claim 2, wherein said foginhibiting compound is incorporated within said emulsion layer in an amount of at least about 0.1 mole per mole of said silver halide and which is substantially larger than normal fog-inhibiting amounts.

4. A method according to claim 3, wherein said foginhibiting compound is a five-membered heterocyclic thione compound carrying an aryl substituent or a fusedon aromatic group.

5. A method according to claim 3, wherein said foginhibiting compound is l-(o-rnethoxy-phenyl)-2-tetrazoline-S-thione.

6. A method according to claim 3, wherein said compound releasing iodide ions in an aqueous medium is a water-soluble iodide.

7. A method according to claim 6, wherein said watersoluble iodide is potassium iodide.

8. A method according to claim 3, wherein said compound releasing iodide ions in an aqueous medium is incorporated within said emulsion layer at a concentration within the range of about 0.01-20 grams per mole of silver halide therein.

9. A method according to claim 3, wherein a compound releasing bromide ions in an aqueous medium, this compound being other than silver bromide, is incorporated within said emulsion layer subsequent to precipitation of the silver halide of said emulsion.

10. A method according to claim 3, wherein the silver halide emulsion layer includes a moistening agent.

11. A method according to claim 10, wherein said moistening agent is a polyalkylene glycol.

12. A method according to claim 3, wherein the silver halide emulsion layer includes polyvinylpyrrolidone.

13. A method according to claim 3, wherein the silver halide emulsion layer includes an aqueous dispersion of a synthetic polymer.

14. A method according to claim 13, wherein said synthetic polymer is polyethyl acrylate.

15. A method according to claim 3, wherein the silver halide emulsion is a silver bromide emulsion or a silver chlorobromide emulsion comprising at least 20 mole percent of silver bromide and containing at most 5 mole percent of silver iodide relative to the total amount of silver halide.

16. A method according to claim 15, wherein the silver halide emulsion is prepared by conversion from a more soluble silver halide emulsion.

17. In a method of producing a colored photographic direct positive image that comprises the steps of:

imagewise exposing a light-sensitive material, which comprises a layer of an unfogged silver halide emulsion of the type that forms a latent image predominantly in the inner part of the silver halide grains,

21 treating the light-sensitive material thus exposed with an aqueous energetic color surface developing liquid in the presence of a color coupler,

overall exposing the light-sensitive material during the treatment stage to actinic light having an intensity lower than a normal daylight intensity to produce a direct positive image in said emulsion layer, and bleaching away the silver developed,

the improvement which comprises incorporating within said emulsion layer subsequent to precipitation of the silver halide of said emulsion a compound other than silver iodide releasing iodide ions in an aqueous medium in an amount sufiicient to effectively increase the density and contrast of the direct positive image in said emulsion layer.

18. A method according to claim 17, wherein a foginhibiting compound, which inhibits fogging during the development stage and which belongs to the class of the heterocyclic thione compounds is incorporated within said emulsion layer.

19. A method according to claim 18, wherein said foginhibiting compound is incorporated within said emulsion layer in an amount of at least about 0.1 mole per 100 moles of said silver halide and which is substantially larger than normal fog-inhibiting amounts.

20. A method according to claim 19, wherein said foginhibiting compound is a five-membered heterocyclic thione compound carrying an aryl substituent or a fusedon aromatic group.

21. A method according to claim 19, wherein said foginhibiting compound is 1-(o-methoxy-phenyl)-2-tetrazoline-S-thione.

22. A method according to claim 19, wherein said compound releasing iodide ions in an aqueous medium is a water-soluble iodide.

23. A method according to claim 22, wherein said water-soluble iodide is potassium iodide.

24. A method according to claim 19, wherein said compound releasing iodide ions in an aqueous medium is incorporated within said emulsion layer at a concentration Within the range of about 0.01-20 grams per mol of silver halide therein.

25. A method according to claim 19, wherein a com- 22 pound releasing bromide ions in an aqueous medium, this compound being other than silver bromide, is incorporated within said emulsion layer subsequent to precipitation of the silver halide of said emulsion.

26. A method according to claim 19, wherein the silver halide emulsion layer includes a moistening agent.

27. A method according to claim 26, wherein said moistening agent is a polyalkylene glycol.

28. A method according to claim 19, wherein the silver halide emulsion layer includes polyvinylpyrrolidone.

29. A method according to claim 19, wherein the silver halide emulsion layer includes an aqueous dispersion of a synthetic polymer.

30. A method according to claim 29, wherein said synthetic polymer is polyethyl acrylate.

3,1. A method according to claim 19, wherein the silver halide emulsion is a silver bromide emulsion or a silver chlorobromide emulsion comprising at least 20 mole percent of silver bromide and containing at most 5 mole percent of silver iodide relative to the total amount of silver halide.

32. A method according to claim 31, wherein the silver halide emulsion is prepared by conversion from a more soluble silver halide emulsion.

33. A method according to claim 17, wherein several silver halide emulsion layers constitute a multilayer color material each layer of which absorbs a particular part of the spectrum and for which layers appropriate color couplers are provided.

References Cited UNITED STATES PATENTS 2,592,250 4/1952 Davey et al. 96-94 3,531,290 9/ 1970 Litzerman 96-107 3,594,172 7/1971 Sincius 96l07 J. TRAVIS BROWN, Primary Examiner W. H. LOUIE, JR., Assistant Examiner US. Cl. X.R. 96107 

